1. Technical Field
The present invention relates to a flyback type switching power supply that produces a second direct current voltage isolated by a transformer from a first direct current power source with a switching operation of a semiconductor switch, and in particular, relates to a control circuit that detects a transformer secondary side output voltage using an auxiliary winding (tertiary winding), and controls the output voltage to a constant voltage.
2. Related Art
A switching power supply is utilized as a power supply for a charger or a power supply of a notebook computer AC adaptor in order to supply a constant voltage to a load. As a heretofore known method, there is a method whereby a voltage detector circuit that uses a photo-coupler for isolating and detecting an output voltage Vo is provided on the secondary side of the switching power supply shown in FIG. 13, and the output voltage is controlled to a constant voltage. The output voltage is constantly monitored with this method, meaning that, even when a sudden load change occurs and the output voltage drops, the drop in the output voltage is swiftly detected, and it is possible to control the output voltage to a constant voltage. However, although it is possible to realize a stable control by directly detecting a secondary side voltage, a power loss constantly occurs in the voltage detector circuit that uses the photo-coupler, which is an impediment to energy saving and increased efficiency.
Meanwhile, as a method of obtaining output voltage information without using the voltage detector circuit that uses the photo-coupler, there is a method whereby the output voltage is controlled to a constant voltage, without directly detecting the secondary side voltage, by equivalently detecting the output voltage from a primary side switching voltage waveform. FIG. 10 shows an example of a heretofore known circuit, shown in JP-A-7-170731 and JP-A-2010-22121, that detects a tertiary winding voltage of a transformer, and controls a direct current output voltage to a constant voltage. As JP-A-2010-22121 is an improved version of JP-A-7-170731, there are differences in the voltage detector circuit.
The configuration and operation of the circuit shown in FIG. 10 are as follows. An alternating current input is converted to a direct current voltage in a rectifying circuit 1, and a direct current voltage Vi smoothed in a capacitor 2 is obtained. With the direct current voltage Vi as an input, a primary winding 6-1 of a transformer 6 is on/off controlled by a MOSFET 7 acting as a semiconductor switch. A secondary winding 6-2 of the transformer 6 is connected to a diode 3, and after rectification, the voltage is smoothed in a capacitor 5, becoming a direct current output Vo. A resistor 4 is a dummy resistor for suppressing a rise in voltage when there is no load or a light load. This is a so-called flyback type switching power supply circuit wherein excitation energy is accumulated in the transformer 6 when the MOSFET 7 is turned on, and there is a charging with the excitation energy from the secondary winding 6-2 to the capacitor 5 via the diode 3 when the MOSFET 7 is turned off. A voltage of a tertiary winding 6-3 of the transformer 6 is rectified and smoothed in a diode 12 and capacitor 11, becoming a power source of a control circuit 13. Also, the control of the output voltage Vo to a constant voltage is carried out by detecting a voltage Vt of the tertiary winding 6-3, which is equivalent to a secondary winding voltage of the transformer 6.
A description will be given, using FIGS. 11 and 12, of the principle of output voltage detection using a tertiary winding. When the MOSFET 7 is turned on, a current Ip flows through the primary winding of the transformer 6, and rises. Next, when the MOSFET 7 is turned off, the current flows from the secondary winding 6-2 through the diode 3, becoming a current Is, is charged by a capacitor Co, and becomes the output voltage Vo. When taking a voltage polarity of the secondary winding 6-2 at this time to be positive, the voltages of the secondary winding 6-2 and the tertiary winding 6-3 when the MOSFET 7 is in an on state are both of negative polarity. When the MOSFET 7 is in an off state, and the current Is is flowing through the diode 3 connected to the secondary wiring 6-2, a voltage Vs of the secondary winding 6-2 is the sum of the output voltage Vo and a forward voltage drop Vf of the diode 3. As a result of this, the voltage Vt of the tertiary winding 6-3 is a voltage proportionate to the turns ratio between the secondary winding 6-2 and the tertiary winding 6-3, and is equivalent to detecting the voltage Vs of the secondary winding 6-2. By detecting the detected output voltage Vt and the current Ip flowing through the primary side MOSFET 7, the output voltage is controlled to a constant voltage. Also, when there is a light load or no load, switching control is carried out so as to suppress standby power by lowering a switching frequency.
As heretofore described, with the method whereby the output voltage information is obtained from the primary side switching voltage waveform, using the method whereby the tertiary winding voltage of the transformer is detected, or the like, without using an output voltage detection method whereby the direct current output voltage is directly detected using the photo-coupler, it is not possible to detect the output voltage unless the semiconductor switch is switching. Because of this, when a sudden load change occurs in a state in which the switching frequency is lowered, and power consumption reduced, when there is a light load or no load, it is not possible, even when the output voltage drops due to the sudden load change, to detect the drop in the output voltage until the next switching. When the switching frequency is lower when there is a light load or no load, the interval between switchings is longer, meaning that there is a problem in that there is a transient large drop in the output voltage between the sudden load change occurring and the next switching.